Method and apparatus for producing an ion-ion plasma continuous in time

ABSTRACT

An ion-ion plasma source, that features a processing chamber containing a large concentration of halogen or halogen-based gases. A second chamber is coupled to the processing chamber and features an electron source which produces a high energy electron beam. The high energy electron beam is injected into the processing chamber where it is shaped and confined by a means for shaping and confining the high energy electron beam. The high energy electron beam produced in the second chamber when injected into the processing chamber ionizes the halogen gas creating a dense, ion-ion plasma in the processing chamber that is continuous in time. A method for creating an ion-ion plasma continuous in time.

FIELD OF THE INVENTION

This invention relates in general to the field of material processingand in particular to the field of using ion-ion plasma source foretching materials.

BACKGROUND OF THE INVENTION

Plasmas are widely used to modify the surface properties of materialsand are now indispensable in etching sub-micron features. These featuresare created using a mask to define the feature, reactive neutrals(radicals) to attack the unmasked areas chemically, and energetic ionsto remove the debris and provide directionality. The plasma providesboth the ions and radicals. In conventional etchers the ions are almostalways positive and are accelerated onto the material by an electricfield. Because most materials being etched are poor conductors, anegative current must accompany the positive ion current, to avoidcharging the surface. The simplest solution is to apply rf fields thatdrive positive ions into the material during one part of the rf cycleand negatively charged particles during the other part. The rf frequencymost commonly used is 13.56 MHz.

Conventional etchers use electromagnetic fields to heat plasma electronsto ionize a background gas, and the plasmas thus formed necessarilycontain large numbers of free electrons. In electronegative gases, someof the electrons attach to the molecules to form negative ions, but theelectrons continue to carry most of the negative rf current because theions are much heavier and less mobile. Moreover, the electrons generatean electrostatic field that prevents negative ions from leaving theplasma. The positive ions and free radicals then do the actual etchingof reactive material in contact with the plasma, while the electronsneutralize any bulk charge left on the surface of the material. Negativeions, while often present, are unsued in conventional etchers.

Using electrons to neutralized positive surface charge works well forlarge-scale features but not small-scale features. This is because thelight and hot electrons flow in all directions, whereas the cold andmassive ions are driven directly toward the material by the applied andself-fields. The ions therefore preferentially strike the bottom of adeep narrow trench, whereas the electrons spread out and strike the sidewalls of the trench. The bottom of the trench thus charges positivelywhile the side walls charge negatively, and this difference in chargegenerates a transverse electrostatic field that deflects ions into theside walls. The side wall then begin to etch and erode, thus deformingthe trench. Deep narrow trenches with straight side walls are thereforedifficult to form with electron-ion plasmas.

One possible solution is to use negative ions rather than electrons toneutralize the surface charge. This requires an ion-ion plasmaconsisting mainly of positive and negative ions but few electrons.Unlike electrons, negative ions flow directly into a material whenaccelerated through a thin, electrostatic sheath adjacent to thematerial. Moreover, negative ions etch as well as, and possibly betterthan, positive ions. In ion-ion plasmas, positive ions flow toward thematerial during one half cycle of the rf field, while negative ions flowduring the other half cycle. However, the rf frequency must now bereduced to 1 MHz or less, to give the massive ions time to respond tothe fields. Also, square rf pulses can be used in place of sinusoidalpulses, to reduce the energy spread of the ions and thereby improve etchselectivity in different materials. Since both current carriers are nowdirected toward the material, deeper and narrower channels can be formedusing ion-ion plasmas. The aspect ratio ultimately achievable is thenlimited by chemical etching from the isotropic radicals alone. Thislimit, which has yet to be reached in present-day etchers, is approachedwith ion-ion plasmas provided the ions are cold and traverse the rfsheath while suffering few collisions.

Conventional electromagnetic discharge sources use hot electrons togenerate a discharge and thus naturally generate electron-ion plasmas.These sources include capacitively coupled discharges, inductivelycoupled discharges, helicons, surface waves, andelectron-cyclotron-resonance reactors. However, if the electromagneticheating fields are turned off, the plasma will convert into an ion-ionplasma in many of the halogen-based gases commonly used for etching.This is because, the dissociative attachment rate rises, in these gases,as the electrons temperature drops, and thus the electrons attach duringthe afterglow (“off” phase) to form negative ions. Pulsing anyconventional source can thus produce an ion-ion plasma late in theafterglow. When the heating fields are on, the electrons are hot andproduce an electron-ion plasma. When the heating fields are off, theelectrons cool, the plasma decays, and an ion-ion plasma eventuallyforms. However, because the electrons are hotter and more mobile thanthe ions, this conversion typically occurs only late in the afterglowwhen the electron density has fallen to several orders of magnitudebelow the ion density. Only at that point are negative ions able toleave the plasma.

The Charged Particle Physics Branch (Code 6750) at the Naval ResearchLaboratory has developed a plasma source for etching called the LargeArea Plasma Processing System (LAPPS). This system is the subject ofU.S. Pat. Nos. 5,182,496 and 5,874,807, both of which are incorporatedherein by reference, in their entireties. This plasma source uses amagnetically confined, sheet electron beam to ionize a background gasand produce a planar electron/ion plasma. Electron beams exhibit highionization and dissociation efficiency of the background gas. Inaddition, the plasma production process is largely independent of theionization energies of the gas or the reactor geometry. Since the plasmavolume is limited only by beam dimensions, the usable surface area ofthe plasma thus can exceed that of other plasma sources.

Although pulsing a conventional plasma source can produce ion-ionplasmas, the technique suffers from several serious limitations. Onelimitation is that hot electrons drive the ion flux during theelectron-ion phase, whereas cold ions drive the ion flux during theion-ion phase. As a result, the ion flux during the electron-ion phaseis orders of magnitude larger than the ion flux during the ion-ionphase. In addition, the ion-ion phase persist for only a brief portionof the afterglow and therefore for an even shorter portion of the totalperiod. The net result is that most of the etching occurs during theelectron-ion phase rather than during the ion-ion phase. The useful dutycycle and efficiency of ion-ion etching from conventional, pulsedsources is thus low. Nevertheless, despite these limitations, pulsedplasmas have been shown to improve etch quality.

Therefore, it would be desirable to produce an ion-ion plasma with ahigh degree of control that is continuous in time.

SUMMARY OF THE INVENTION

Disclosed is an ion-ion plasma source featuring a processing chambercontaining a large concentration of halogen or halogen-based gas. Asecond chamber is coupled to the processing chamber and features anelectron source which produces a high energy electron beam. The highenergy electron beam is injected into the processing chamber where it isshaped and confined by a means for shaping and confining the high energyelectron beam. The high energy electron beam produced in the secondchamber when injected into the processing chamber ionizes the halogengas creating a dense ion-ion plasma in the processing chamber that iscontinuous in time.

Also disclosed is a method for creating an ion-ion plasma continuous intime comprising a processing chamber containing a large concentration ofat least one halogen gas and a second chamber coupled to the processingchamber. Creating a high energy electron beam in the second chamber,injecting the high energy electron beam into the processing chamber,shaping the high energy electron beam injected into the processingchamber with a magnetic field. Wherein the high energy electron beaminjected into the processing chamber ionizes the halogen gas creating adense ion-ion plasma in the processing chamber that is continuous intime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for producing an ion-ion plasma continuouly intime.

FIG. 2 shows an example beam source for producing a high energy electronbeam.

FIG. 3 shows a second example beam source for producing a high energyelectron beam.

DETAILED DESCRIPTION

Referring to the figures wherein like reference numbers denote likeelements, FIG. 1 shows an example embodiment of CIIPS, an apparatus forproducing and ion-ion plasma 100 that is continuous in time.

As shown in FIG. 1, plasma source 100 features a processing chambercomprising 101, having therein a large concentration of a halogen-basedgas. A second chamber 111 is coupled to the processing chamber 101 andcontains therein an electron source which provides a high energyelectron beam 112, in the second chamber 111. Processing chamber 101features means for shaping the high energy electron beam which in theexample embodiment is a longitudinal magnetic field applied to thesurface of the chamber wall in the direction of propagation. Thelongitudinal magnetic field generally is externally generated, andapplied to keep the beam from expanding and striking the substrate, andto keep the beam current density, and thus the plasma densityapproximately constant as the beam propagates, and to retard the outwardflow of plasma electrons. In an example embodiment, the magnetic fieldis produced by positioning magnetic field coils, or possibly permanentmagnets, along to direction of electron beam propagation.

In operation the high energy electron beam 102 produced in the secondchamber 111 is injected into the processing chamber 101 and is confinedtransversely by the magnetic field. The confined high energy electronbeam 102 in the processing chamber 101 ionizes the gases creating adense, ion-ion plasma 103 in the processing chamber. The ion-ion plasmais produced continuously in time. The high energy electron beam 102injected into the processing chamber 101 creates a ion-ion plasma 103 bydissociating the molecules of the halogen-based gas into a group of coldplasma electrons, free electrons and positive ions 109. Specifically,the plasma electrons and positive ions 109 are created throughionization, while neutral radicals are created through thedisassociation of the halogen molecules. The cold free electrons (1 eV)created in the plasma attach to halogen molecules to form negative ions108. This produces a dense plasma 103 that features a largeconcentration of positive ions 109, negative ions 108 and neutralradicals 110.

The processing chamber features two or more planar substrate stages (notshown). These substrate stages are closely spaced to provide room forthe electron beam to pass between them. The material to be processed 107is placed on one or more of the stages and an rf voltage 105 is appliedas necessary to accelerate the ions, 108 and 109 onto the material beingprocessed 107.

The distance from the electron beam 102 to the substrate stage providesadditional control over the particle fluxes, separate from the beam andgas parameters. Typically, the stages sit 1 cm or more from the electronbeam 102 in order to prevent the beam 102 from striking the materialbeing processed 107.

CIIPS employs a magnetically confined sheet electron beam to ionize anddissociate a background gas. CIIPS produces a continuous ion-ion plasmarather than an electron-ion plasma, by using a gas mixture containing alarge concentration of halogen gas with a large attachment cross sectionat electron energies below 1 eV. Candidate gases include SF₆, Cl₂ andF₂.

The high energy electron beam is confined transversely by a longitudinalmagnetic field to maintain plasma uniformity over a large area, toprevent the beam from striking the substrate, and to reduce the flux ofplasma electrons to the substrates. These features minimize the loss ofelectron energy.

The electron beam may be produced in a chamber separated from theprocessing chamber by differential pumping as indicated in FIG. 1. Thisfeature helps to minimize gas contamination and improves processingcontrol. The high energy electron beam within the second chamber isapproximately 2000 eV. This energy level can vary depending on the gaspressure and the system length.

In a preferred embodiment the high energy electron beam employed by thedisclosed ion-ion method has an energy level approaching 2000 eV. Assuch, the ionization energies of the gases can differ widely, since theelectron beam has sufficient energy to ionize and dissociate any and allgases. Moreover, the ionization and disassociation rates of a given gasconstituent are largely determined by the concentration of thatconstituent for a given electron beam, which allows the processingchamber to be populated with a wide mixture of halogen gases. Bycontrast, prior methods were often restricted to the use of halogengases with similar electron bond strength. In the present invention, theoption of varying the gas mixture provides direct control over theplasma constituents and the plasma chemistry.

The beam energy is nominally a few keV or less, the beam current densityis typically 0.1 A/cm² or less, the gas pressure in the processingchamber is typically 50 mtorr, and the magnetic field along the beam isaround 200 G. The beam is normally a few cm thick and arbitrarily wide,as determined by the chamber size and application. The magnetic field isapplied to keep the beam thickness approximately constant over the beamrange. For the parameters specified the beam range is 1 m or more, andthe ion density produced is as high as 2×10¹² cm⁻³. CIIPS can thusgenerate dense, uniform, ion-ion plasmas over processing areas as largeas 1 m² or more.

In a preferred embodiment the electron beam is shaped into a thin sheet.The sheet beam can be produced in a variety of ways, and two methodshave been successfully demonstrated and are shown in FIGS. 2 and 3.

FIG. 2 shows an example beam source used to produce a high energyelectron beam. Referring to FIG. 2, a high-voltage discharge 202 isstruck between a long, hollow cathode 201 and a slotted anode 203. Aportion of the discharge current emerges in the form of an energeticelectron beam 204 that passes through the slot into the processingchamber, while the remainder of the discharge current flows to the anode203.

FIG. 3 shows a second example beam source for producing a high energyelectron beam. Referring to FIG. 3, electrons are extracted from a denseplasma or other electron source 301 and then accelerated by a highvoltage 305 applied to a nearby grid 302 or slot 303. Both methods arecapable of generating electron beams of the required energy and currentdensity at gas pressures below 300 mtorr.

Referring again to FIG. 1, the magnetic field is applied to the electronbeam 102 to prevent the beam from striking the stage or the materialbeing processed 107, and to keep the beam current density approximatelyconstant over the propagation length, and to reduce the outward flow ofplasma electrons. A field of around 200 G keeps the beam gyroradiusunder 1 cm, which is generally sufficient for CIIPS. The field stronglyretards the flow of plasma electrons but has little effect on themassive ions, and as a result, negative ions can escape the plasma andstrike the substrate 107 more easily than in other plasma sources.

As the electron beam 102 collides with the halogen and other gasmolecules, it generates ions, electrons, and radicals through ionizationand dissociation. At the same time, gas flow keeps the gas cold and thedegree of ionization and dissociation low. The plasma electronstherefore cool rapidly and attach to form negative ions, therebyproducing a weakly ionized but dense plasma 103 consisting mainly ofpositive 109 and negative ions 108 and neutral radicals 110. As theseparticles diffuse out of the plasma, they etch any reactive materialthey contact.

The etch rate may be increased by placing the material on a stage towhich rf is applied at a frequency approximately ≦1 MHz. The rf voltageincreases the energy of the ions (to typically 20 eV or more) strikingthe material. At low gas pressure, the rf sheath is thinner than the ionmean free path, and thus isotropic radicals together with energetic andhighly anisotropic, positive and negative ions strike the material. Aspreviously noted, the ion flux from an ion-ion plasma is much smallerthan that from an electron-ion plasma of the same density, and thus theetch rate is smaller as well. The reduction in etch rate is partiallyoffset by a reduction in substrate heating, and the etch rate can beincreased to some extent by raising the beam current to increase theplasma density.

The method for creating an ion-ion plasma continuous in time comprises aprocessing chamber containing a large concentration of at least onehalogen gas, and a second chamber coupled to the processing chamber. Themethod includes creating a high energy electron beam in the secondchamber and injecting the high energy electron beam into the processingchamber. After the electron beam is injected into the chamber the nextstep is shaping the high energy electron beam injected into theprocessing chamber with a magnetic field. The high energy electron beaminjected into the processing chamber ionizes the halogen gas, creating adense ion-ion plasma in the processing chamber that is continuous intime.

The high energy electron beam injected into the processing chambercreates a ion-ion plasma by dissociating the molecules of the halogengas into a group of cold plasma electrons, free electrons and positiveions, and the cold free electrons created in the plasma attach tohalogen molecules forming negative ions producing a dense plasmacomprising a large concentration of positive and negative ions andneutral radicals. The high energy electron beam within the secondchamber is approximately 2000 ev. The processing chamber contains amultitude of halogen gases.

The high energy electron beam is shaped and confined by a magnetic fieldwhich provides uniformity over a large area and minimizes the loss ofelectron energy.

Although this invention has been described in relation to the exemplaryembodiment's thereof, it is well understood by those skilled in the artthat other variations and modifications can be affected on the preferredembodiment without departing from scope and spirit of the invention asset fourth in the claims.

1. An ion-ion plasma source comprising: a processing chamber comprisinga large concentration of halogen gas; a second chamber coupled to theprocessing chamber; an electron source for producing a high energyelectron beam in the second chamber; means for confining the high energyelectron beam; wherein the high energy electron beam produced in thesecond chamber is injected into the processing chamber and is confinedby the means for confining, the confined high energy electron beam inthe processing chamber ionizes the halogen gas creating a dense ion-ionplasma in the processing chamber that is continuous in time.
 2. Thesource of claim 1 wherein the high energy electron beam injected intothe processing chamber creates a ion-ion plasma by ionizing anddissociating the gas molecules into a group of cold plasma electrons,free electrons and positive ions, and the cold free electrons created inthe plasma attach to halogen molecules forming negative ions producing adense plasma comprising a large concentration of positive and negativeions and neutral radicals.
 3. The source of claim 1 wherein the meansfor confining the high energy electron beam is a longitudinal magneticfield applied in the direction of propagation.
 4. The source of claim 1wherein the processing chamber contains mixture of multiplehalogen-based gases and non-halogen gases.
 5. The source of claim 1wherein the processing chamber contains a gas mixture comprising a largeconcentration of halogen-based gas with a large attachment cross sectionat electron energies below 1 eV.
 6. The source of claim 1 wherein theprocessing chamber comprises at least one substrate stage for disposingthe material to be processed, and comprising means to vary the distancebetween the at least one substrate stage and the high energy electronbeam.
 7. The source of claim 6 wherein a longitudinal magnetic fieldapplied to the processing chamber shapes and confines the high energyelectron beam and reduces the plasma electron flux to the substrates. 8.The source of claim 1 wherein the high energy electron beam within thesecond chamber is approximately 2000 ev.
 9. A method for creating anion-ion plasma continuous in time having a processing chamber containinga large concentration of at least one halogen gas and a second chambercoupled to the processing chamber; the method comprising: creating ahigh energy electron beam in the second chamber; injecting the highenergy electron beam into the processing chamber; and shaping the highenergy electron beam injected into the processing chamber with amagnetic field; wherein the high energy electron beam injected into theprocessing chamber ionizes the halogen gas creating a dense ion-ionplasma in the processing chamber that is continuous in time.
 10. Themethod of claim 9 wherein the high energy electron beam injected intothe processing chamber creates a ion-ion plasma by dissociating themolecules of the halogen gas into a group of cold plasma electrons, freeelectrons and positive ions, and the cold free electrons created in theplasma attach to halogen molecules forming negative ions producing adense plasma comprising a large concentration of positive and negativeions and neutral radicals.
 11. The method of claim 9 wherein the highenergy electron beam within the second chamber is approximately 2000 ev.12. The method of claim 9 wherein the processing chamber contains amultitude of halogen gases.
 13. The method of claim 9 wherein the highenergy electron beam is shaped and confined by a magnetic field whichprovides uniformity over a large area and minimizes the loss of electronenergy.
 14. A system for plasma etching comprising: a processing chambercontaining at least one processing stage; a substrate material to beprocessed disposed on the processing stage; a masking material appliedto the substrate material to coat selected areas of the substratematerial while leaving other areas of the substrate material exposed;means for producing an ion-ion plasma continuous in time within theprocessing chamber wherein said ion-ion plasma comprises, negative ions,positive ions and neutral radicals; wherein the ion-ion plasma etchesthe exposed area of the substrate material and the strength and polarityof the walls of the processing chamber is altered to control the rate atwhich charge is accumulated on the surface of the substrate material.15. The system of claim 14 wherein the ion-ion plasma source comprises:a processing chamber comprising a large concentration of halogen gas; asecond chamber coupled to the processing chamber; an electron source forproducing a high energy electron beam in the second chamber; means forconfining the high energy electron beam; wherein the high energyelectron beam produced in the second chamber is injected into theprocessing chamber and is confined by the means for confining, theconfined high energy electron beam in the processing chamber ionizes thehalogen gas creating a dense ion-ion plasma in the processing chamberthat is continuous in time.